Corrosion Under Insulation

Michael Cordon

David Harvey

Matthias Kiesel

ChE 597

Team 2G

December 4, 2017

Executive Summary

Corrosion under insulation (CUI) is a major safety threat to chemical processes with pipes

and pressure vessels for moving and storing fluids where anions and cations are transported via

water to pipe surfaces to corrode away pipe surfaces. Since insulation is necessary for minimizing

energetic losses to the environment and therefore can significantly affect the economic feasibility

of processes, insulation is required for economic optimization yet insulation hinders the ability to

visibly monitor corrosion to external pipe surfaces. CUI is responsible for up to 60% of corrosion

related incidents and the direct damage done by CUI is estimated to be $276 billion per year as of

2005. This report discusses CUI causes and types of damage to pipelines, different techniques of

CUI detection, and the strengths and weaknesses of each of these techniques. It was determined

that a combination of several existing techniques is required as the most efficient way to prevent

and mitigate CUI from both an economic and time standpoint. Finally, this report looks at several

experimental techniques that are being studied for the improvement of CUI detection.

Besides physically removing the entire swaths of insulation from pipes which is extremely

costly, four other technologically sophisticated possibilities are discussed here. Neutron

backscattering is an inexpensive screening method for detecting moisture in insulation while X-

Ray radiography can be used for real time scanning in small appliances and also in more complex

settings to create a detailed picture of the pipe or vessel. Ultrasonic thickness measurement (UTM)

uses soundwaves to measure the thickness of the pipe and is especially helpful when dealing with

difficult geometries as a secondary measurement method. Pulsed eddy current (PEC) utilizes

electromagnetic effects to generate an eddy within a flowing fluid and then measures decay rate

of the magnetic field to calculate the wall thickness. Combining these methods efficiently is

essential for cost effective CUI detection with recommendations given at the end of this report for

the most effective combination. Research in the field of CUI aims at providing steady state

screening methods to eventually make in person measurements unnecessary. These techniques

could significantly improve the cost effectiveness of CUI detection and reduce the risk of incidents

caused by CUI.

Introduction

Insulation has been widely used for decades to maximize energy conservation and maintain

temperatures in flowing fluids. However, effective insulation requires complete encapsulation of

metal pipe and tank walls which can cause a hidden corrosion danger under said insulation. These

same concerns can also be applied to fireproofing materials which protect pipes and tanks in case

of fire1. Corrosion under insulation (CUI) is becoming a common safety issue as metal wall failures

have become increasingly prevalent within aging chemical plants, resulting in fluid releases from

piping and tanks. This in turn can result in the release of flammable compounds and explosion and

fire hazards. From an economic standpoint, CUI is responsible for roughly 40-60% of all pipe

maintenance costs which can total around 10% of a facility’s overall maintenance budget if left

unchecked2. Overall, this issue has been identified to cost as much as $276 billion per year for the

chemical and petrochemical industries, with up to 60% of corrosion-related incidents occurring on

pipelines.

Traditional CUI is caused by the aqueous intrusion into insulation (or fireproofing1)

surrounding process pipes. As water diffuses inwards, anions (alkali, alkaline) and/or cations

(halogens, nitrates, sulfates, etc.) dissolve and diffuse into water as it moves towards pipe surfaces,

leading to the eventual buildup of highly corrosive solutions in close proximity to pipe walls. These

corrosive solutions can be of much lower (or higher) pH than bulk fluid measurements and will

begin to break down protective coatings given enough time and high anion or cation

concentrations. Higher temperatures also lead to higher corrosion rates until water evaporation

eventually occurs. Upon additional water intrusion, anions and/or cations in the newly made

solution continue their localized attacks on the pipe’s structural integrity3.

Localized attacks on pipe walls typically occur through either pitting or stress-corrosion

cracking (SCC). Pitting refers to the localized attack of a corrosive agent on passivating films

which first ruptures protective passive films prior to forming holes in metal walls, allowing for

small fluid leaks and releases. Released corrosive fluids typically result in significantly more

severe pipe damage as the corrosion spreads radially, increasing the pit size over time4. Stress-

corrosion cracking of alloys and metals (stainless steel by chlorides, alkali and/or nitrates on steel,

ammonia on copper) is often caused by conditions well below those of traditional fracture

formation. Microscopic cracks permeate throughout the piping due to only mildly corrosive

conditions but remain imperceptible to the human eye until the pipe becomes quite brittle and

cracks. This behavior is caused by localized attack from corrosive agents and can happen both on

exterior and interior pipe surfaces4.

CUI safety incidents and near misses caused by the factors mentioned above have been

studied for over 50 years now with many explicit safety incidents occurring globally. Notable

incidents include the oleum release at a DuPont plant in 2010 caused in part by pitting from CUI5,

the hydrocarbon explosion and resulting fire at a Williams Olefins petrochemical plant in 20136,

and an explosion at a DOW light hydrocarbon plant in 20087. These incidents are by no means

limited to the United States though with lists of CUI incidents occurring under EU and OECD

jurisdiction8 or in specific countries such as Japan which date back to at least 19658. These

incidents are a conglomeration of pitting and SCC incidents and often result in explosions and/or

fires. Statistically, the most commonly released compounds are hydrogen and hydrocarbon

products which easily lead to explosions and fires and are arguably the most dangerous component

of CUI incidents8.

Some preventative measures are known and globally used to mitigate CUI concerns

overall. To some extent, these preventative measures are dependent on the material of construction

of the pipes and tanks in question. Organic coatings are often applied to external metal surfaces to

hinder CUI through unfavorable interactions between the organic film and the aqueous solutions.

However, these films degrade over time with lifetimes lasting from 5-13 years on average2. Since

this is often a lower lifetime than the equipment they protect, the organic coating must be

maintained and reapplied periodically1. As the film breaks down, the potential for pit or crack

formation increases significantly. In addition to the organic films, aluminum foil has been widely

used as a protective anionic layer to prevent against cationic attack. This preventative measure is

commonly used due to its immediate accessibility, low initial cost, and simple installation.

Aluminum can also be applied through thermal spray aluminum which applies an aluminum coat

of increased thickness to withstand more severe CUI environments for longer durations.

Improvements in the inherent design of insulation and the associated appropriate installation to

eliminate water presence and buildup is also a critical preventative method2. This combination of

preventative techniques serves to lengthen the lifespan of pipe and tank walls from CUI but is not

useful for directly detecting piping sections where CUI is occurring. Additional details and risk

analysis equations have been published by the American Petroleum Institute for analyzing CUI

requirements and concerns9.

Objective of Report

The objective of this report is to understand the scale and severity of corrosion under

insulation in the chemical and petrochemical industry with regards to both the inherent safety

concerns and the economic costs. Current detection methods are discussed for identifying potential

CUI occurrences and analyzing the severity of these situations. Strengths and weaknesses of each

technique will be discussed including detection limitations, applicability to tanks, pipes, or both,

and their relative costs. Additional discussion will focus on the desired attributes of an ideal CUI

detector along with the research needs necessary to support the development of this type of

technology. Then, recommendations are provided for which current techniques should be used in

tandem to best collect CUI data and prevent potential CUI incidents from occurring.

Literature Review

CUI has become increasingly concerning as chemical manufacturing facilities age. CUI

incidents discussed above focus primarily on relatively recent release scenarios within the United

States. Here, the Chemical Safety Board acts as a primary source for individual, full-scale CUI

incident investigations5,10 with news report and journals providing more easily digestible

stories6,11. In addition, companies will occasionally publish their own findings and reports on

release or near-miss scenarios7. Since CUI is a global issue though, foreign governmental and

regulatory agencies from both Europe and Asia have also investigated CUI incidents looking for

general trends in CUI-related incidents8,12.

There is a wide body of literature, both peer-reviewed and otherwise, looking at the various

types and underlying causes of CUI in industrial applications. Notable publications include reports

from the National Board of Boiler and Pressure Vessel Inspectors3, NACE International13, and

ASM International4 written on corrosion methods, detection, and mitigation. These sources go into

detail regarding how CUI rates are affected by a number of variables including temperature,

humidity, and anion and cation content. More simplified approaches come from professional

presentations13,14 or shorter articles15. This provides a general overview of CUI issues and the

parameters known to increase CUI rates, forming a broad understanding of CUI concerns.

CUI risk-based inspection technology guidelines have been previously published by NACE

International1 and by the American Petroleum Institute9 since many CUI incidents occur in

petrochemical applications. Comprehensive reviews and presentations are available for predicting

CUI occurrences16 and predicting areas likely to experience more rapid CUI rates15,17. Economic

comparisons for some general CUI prevention methods were reported by ExxonMobil in 2005

with a focus on simple preventative measures2. Each of the budding detection methods discussed

below (neutron backscatter13,15,17, radiography13,15,17, pulsed eddy current18–20 , and ultrasonic

thickness measurement21) have detailed technical specifications reported along with their strengths

and weaknesses which are summarized in Table 1. Newer techniques currently being developed

for CUI applications include microwave detection22 and fiber optic acoustic emission sensing23,24

with sources providing both details on each technique’s technical capabilities along with how they

can be utilized for identifying minute cracks in pipes.

Current Detection Methods Overview

Some generalized preventative measures for protecting pipes and tanks from CUI are

already in place in many chemical plants which include the implementation of organic coatings on

pipe surfaces, thermal spray aluminum or aluminum foil wrapping for protection against cathodic

attacks, and the use of personnel protection cages in place of insulation when the primary goal of

insulation is personnel protection rather than energy conservation and temperature consistency.

Next, detection methods will be discussed as ways of identifying potential CUI occurrences prior

to failure scenarios occurring. Each method is discussed with its strengths and weaknesses which

are collected together in Table 1 below.

Table 1: Qualitative comparison of CUI detection method strengths and weaknesses.

Technique Damage to Insulation

Detection of Hotspots

Screening Ability

Applicable to Vessels

Applicable to difficult geometry

Insulation Removal

Not Applicable

Yes No Yes Yes

Neutron Diffraction

No No Yes Yes Yes

X-Ray Scanning

No Yes No No Limited

Ultrasonic Thickness Measurement

Minor Damage

No Limited Yes Yes

Pulsed Eddy Current

No No Yes Yes No

Method 1: Visual Inspection of Metal Surfaces

The most obvious method for detecting CUI instances occurring on tank surfaces or

sections of piping is through direct physical inspection. By necessity, this means removing the

insulation or fireproofing from the suspected section of metal wall and physically analyzing the

potentially affected areas. This most likely coincides with a halt in operations since the temperature

could not be easily maintained while the insulation is relocated. Insulation removal is a slow and

therefore expensive process and must be reinstalled with great care to ensure that water remains

outside of the insulation and away from the pipe surfaces. This method is the most direct means

of detection but may lead to false negatives since SCC fissures and cracks (and even pits in some

cases) are often invisible to the naked eye. This method also requires significant time and

understanding for determining which pipes are most likely to experience CUI incidents and

requires developing and implementing a rigorous preventative maintenance program to ensure that

release scenarios do not occur. For instance, more regular visual inspections would be required for

pipe and tank sections that are exposed to the weather, especially in locales with higher

precipitation rates. This doubly applies in naturally humid climates where water accumulation

occurs more readily or in particularly hot environments where CUI occurs more rapidly. This is

often considered the most expensive method for determining and predicting CUI incidents2.

Method 2: Neutron Backscatter

Neutron backscatter is a technique that can be used to detect moisture in the insulation of

pipes and vessels13,15,19. It operates by sending a beam of high energy neutrons through a pipe or

vessel. If there is moisture in the insulation, the hydrogen atoms in the moisture will reduce the

energy of the neutrons and they backscatter in such a way that they come back to the detector. The

detector counts the number of low energy neutrons where the number of counts is proportional to

the amount of water in the insulation13. The neutrons are produced using a radiation source, and

because the high energy neutrons do not show up on the detector it gives an accurate measure.

This technique was first introduced for monitoring liquid levels in ammonia tanker cars as it detects

any hydrogen-rich solutions17.

The biggest drawback of this technique is that it does not detect corrosion directly. It can

only indicate where corrosion could occur more readily13. This means that further investigation of

direct methods is necessary to determine if corrosion is occurring. Additionally this technique can

give false positives for corrosion depending on the conditions and must be calibrated based on dry

samples14,19. The advantage is that this technique is very fast, versatile, and small13,15,19. This

technique can be used to scan a lot of pipe quickly and can give potential problem areas for CUI13.

Because it is light, it can be used without additional scaffolding13,19. Additionally, this technique

can be used for both piping and vessels15.

Method 3: X-Ray Radiography

Radiography is another nondestructive technique used for the detection of CUI.

Radiography utilizes x-rays to create images and profiles of the piping. There are 3 common

techniques for radiographic use of x-rays: real-time radiography, computed radiography, and

digital detector arrays. Real time radiography uses an x-ray source or radiation source with a

detector to create a profile of the outside of the pipe13,14. Computed radiography and digital detector

arrays work slightly differently by creating a profile of the entire pipe. These can be used to not

only detect corrosion but measure corrosion levels quantitatively13. Computed radiography uses

an imaging plate to digitalize the images from the x-ray source19. The image plate uses a “photo

stimulable phosphor” and can produce an image in between 1-5 minutes13. Digital detector arrays

are similar to computed radiography only instead of a film or plate, flat-panel detectors are used

to detect the x-ray radiation from the source. This method is quite sensitive so they require less

radiation than a film dependent technique.

These techniques have similar pros and cons compared to each other. All of them provide

imaging that is fast and quickly analyzed19. They also require radioactive sources and are more

accurate on smaller pipe diameters as the signal is stronger13,19. Additionally they are more difficult

than other techniques to use in tight spaces because they require access to both sides of the piping

and thus are difficult to use on vessels or even pipes located in spots that are hard to reach14. Each

of these techniques can provide quantitative measurement of wall loss within a pipe, however

computed radiography and digital detector arrays give more detailed and sharper images which

translates to more powerful information. When deciding between these techniques, cost and time

are the distinguishing factors between these similar x-ray techniques.

Method 4: Pulsed Eddy Current

The pulsed eddy current (PEC) method relies on the induction of an eddy current within a

pipe caused by a magnetic field generated by an excitation current. Figure 118 shows the circular

excitation coil wrapped around two anisotropic magneto-resistive sensors (AMR) on top of the test

object such as a pipe. The test object remains in a multi-layered state, meaning that the PEC method

does not require the removal of insulation or protective coatings from the pipe surfaces though it

does require the pipe to be filled with fluid. Each of these layers may have a different thickness,

electrical conductivity, and relative magnetic permeability. For theoretical simplicity, the test

object is assumed to be planar if the excitation coil diameter is smaller than the pipe diameter18.

During PEC testing, the excitation coil generates an eddy current in each of the mentioned layers

until a steady state flow is achieved. The excitation then stops and the sensor measures the decay

of the magnetic field generated by the eddy current18. The desired output is either the maximum

peak height for non-conductive pipe materials or the decay rate of the magnetic field for conductive

pipe materials20. Additional repeat measurements can be obtained from changing the polarity to

generate either positive or negative excitation currents18. Plugging in the known parameters and

the measured magnetic field behavior, the wall thickness can then be calculated explicitly.

PEC application can be limited by insufficient information on the magnetic and electrical

properties of each material of construction. Thicknesses of protective coatings and insulation must

also be precise for reliable measurements. Therefore, this is not an infallible technique since this

information must be accurately supplied. Furthermore, the resulting signal is averaged over an area

of pipe and ultimately assumes an even wall thickness distribution in this area, meaning that the

technique cannot locate small points of acute corrosion. This also limits its ability to detect CUI

on equipment fittings which may have difficult geometries19. Overall, PEC is a noninvasive and

cheap method to accurately measure wall thicknesses of industrial piping given adequate constants

which does not require removal of insulation or protective coatings.

Figure 1: Structure of a PEC measurement device18.

Method 5: Ultrasonic Thickness Measurement

Ultrasonic thickness measurements (UTM) make use of the fact that soundwaves have a

characteristic velocity in each material and additionally are reflected on boundaries between

different materials21. UTM operation occurs by emitting ultrasonic soundwaves into a pipe and

measuring multiple echoes as the soundwave passes through the leading and lagging edges of each

pipe wall. The time required to record these echoes can then be used along with the speed of sound

through the fluid and the pipe material of construction to calculate the wall thickness for both sides

of the pipe using the following equation: 𝑙 =𝑐𝑡

2. Here, l is the wall thickness of the pipe, c is the

celerity of sound through the metal, and t is the time it takes to measure the echo. These

measurements are conducted in a pulse/echo mode in order to reduce the noise in the measured

signal and can be repeated rapidly.

Important measurement parameters include the soundwave wavelength (ranging between

50 kHz and 20 MHz) and the transducer type. Longer wavelengths may be required for thicker

pipes due to penetration depth concerns, but short waves yield more accurate measurements

overall. Therefore, an optimal wavelength can be determined for a given starting thickness. More

important is the choice of the transducer which emits the soundwaves and receives the echoes.

Transducer choice and transducer mode will alter the measured timespan which is the critical

measurement for determining the wall thickness. The simplest method measures only the time

between the initial pulse and the first backwall echo. More advanced methods use a transducer

which is slightly lifted off the surface to measure the time between the echo from the soundwave

first entering the wall and the first backwall echo or the time between two backwall echoes as the

soundwave bounces back multiple times21. These measurements therefore require accurate time

measurements and accurate speed of sound values. Time measurements can be improved using

multiple repeat measurements while the speed of sound through the metal or alloy may be

generated by measuring the pipe prior to initial insulation placement and flow operations but held

at the operating temperature (temperature will alter the speed of sound through the material).

The practical strength of UTM lies in its quick scanning nature and response time. The

technique can also handle difficult geometries with minimal difficulty and relays accurate wall

thickness measurements. However, two major weaknesses should be mentioned. First, the speed

of sound through the wall is a function of temperature which much be rigorously accounted for to

avoid systematic errors in wall thickness measurements. Since temperatures will vary throughout

the plant, this may introduce sampling difficulty. Second, UTM typically requires direct contact

between the transducer and the pipe wall, meaning that the surrounding insulation must be

punctured prior to collecting measurements. This makes it critical to reseal insulation punctures

appropriately to avoid water ingress and therefore mitigate higher corrosion rates after

measurement. This resealing leads to higher costs with increasing sampled area, limiting the

technique applicability.

Current Research Directions

The current techniques for detection of CUI typically are very accurate for a short distance

of piping or are a screening method that can cover a lot of piping quickly while sacrificing accuracy

or quantitative information. An ideal detection method would both give accurate information in

terms of the location and severity of corrosion and monitor large sections of piping in real time.

Several experimental techniques are being researched for this purpose.

One possible technique for this purpose is the use of microwave signals through insulation

to detect water in the insulation. Microwave antenna can be placed throughout the insulation

creating a network of water detectors within the insulation. If water is in the insulation it absorbs

some of the microwave energy, causing an amplitude difference that can be tracked to roughly the

spot of the wet insulation22. This technique would however still only screen for water in insulation

much like neutron backscatter and thus would still only act as a screening technique.

Another promising area of research uses fiber optic acoustic emission sensors to detect

corrosion in piping. An acoustic emissions sensor can be calibrated to give different signals based

on the acoustic inputs. As pipes corrode the signal through the pipes tends to change23,24. By

monitoring the frequencies, the corrosion can be tracked. While in its current state, this technology

can’t pinpoint the corrosion, it can be used to find problem areas in piping networks.

By pairing these future experimental technologies that can monitor pipelines with the

current technology that can be used to pinpoint locations of CUI, problems can be prevented before

they occur. Additionally, better prediction methods and models for CUI are being developed to

assist in determining high risk pipes16. By predicting areas that are most likely to suffer from CUI,

additional monitoring can be done in these areas. Implementing the technology based on risk

allows for more efficient allocation of resources that could prevent CUI incidents. This also can

allow for better planning of mitigation strategies especially in cases with toxic or flammable

chemicals.

Recommendations

In the previous sections, the different techniques for detecting CUI were examined. Clearly

each technique has its own benefits and disadvantages and is optimally used for different purposes

within the field of detecting CUI. We recommend an approach with many different layers of

detecting CUI. First of all, areas susceptible to CUI have to be identified and prioritized for

detection measures. This step is not only performed within plant planning but also updated

throughout the plant’s lifetime. Susceptible areas should be updated as screening information

becomes available over time. Additionally, when changes to the process or the equipment are

made, CUI should be considered within a management of change procedure.

The second step in detecting CUI consists of utilizing scanning techniques. Scanning

techniques are cheap and fast in comparison to other techniques, but at the cost of quality of

information. Probably the cheapest and fastest technique is neutron backscatter, which has the

ability to scan long stretches of piping or large vessels. Because of the previously discussed

disadvantages of this technique, it should not be the only technique utilized but it remains as a

strong example of a scanning technique to be used in the second step of the recommended

approach. Other examples of scanning techniques that could be used are PEC and real-time

radiography. These techniques could also be used to scan depending on the piping layouts and

structures, as well as the cost to perform each technique.

The third step is to utilize more expensive quantitative techniques on problem areas

identified by the scanning techniques to characterize the corrosion in these areas. Because

changing equipment is expensive, quantitative data will allow for the exclusion of false positive

results and determine the severity of CUI. The most reliable method for this task would be X-ray

radiography to obtain a more complete picture of the extent of CUI. Unfortunately, this is not

applicable to big vessels and reactors as both sides of the equipment have to be accessed. A cheaper

measurement method is UTM. UTM is more applicable to difficult geometries and vessels but

eventually has to be applied in a narrow grid to investigate CUI hotspots. Therefore, UTM is

recommended for in-depth examinations since it is accurate and comparably easy to apply.

Removal of insulation does not give strong advantages over any of the methods in this paragraph

due mostly to its high cost; however, during planned plant shutdowns and revisions, removal of

insulation can give accurate CUI measurements. Because each technique has strengths and

weaknesses that are specific to the application, it is recommended that each technique be examined

in both the scanning and quantitative categories before a selection is made.

Conclusions

Corrosion under insulation will continue to occur for the foreseeable future, making

preventative maintenance and CUI detection methods of critical importance. Because insulation is

a key part of the energy conservation strategy in many chemical processes, it will remain an

important challenge in the near future. When combined, current techniques can be used in both

screening and quantitative diagnostic capacities. By utilizing cheaper screening techniques such

as neutron backscatter on large sections of pipe to identify areas of concern and then following up

with more quantitative techniques that can be used to pinpoint and diagnose the damage to piping

and tanks, damage due to CUI can be effectively detected. Through regular screening and data

collection, effective predictions of problem areas can be used to determine the likelihood and

frequency of necessary maintenance based on the surrounding conditions.

Experimental research in the area of CUI is moving toward a more ideal system where

larger sections of pipe can be monitored in real time to pinpoint points of corrosion. Work is still

needed to quantitatively track the corrosion of large sections of pipe in real time and it will be a

long time before the ideal corrosion tracking systems are developed. By embracing the newest

prediction algorithms for corrosion and utilizing the current technology, major incidents due to

corrosion under insulation can be more readily prevented in the future.

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